This invention relates to an improved industrial process for the manufacture of dialkyl xanthogen formates with the general formula: ##STR1## where R and R' are alkyl groups that can be identical or different, straight chain or branched, and that contain from 1 to 6 carbon atoms. The hydrogen groups may also be replaced by halogen atoms, cyanide groups or other nitrogen compounds. Unsaturated groups have also been shown to be useful in mineral reagents, pesticides and as accelerators.
These compounds have been described as useful collectors in a number of expired patents, but there are no process patents and only laboratory preparations are described in the literature. The main patents on the subject and the examples on synthesis of these products are U.S. Pat. Nos.:
Douglass 1,652,099 (1927),
Fischer 1,684,536 (1928),
Fischer 2,608,572 (1952) and
Twiss et al, (U.K. Pat. No.) 353,871 (1931).
Generally, these patents disclose laboratory procedures for preparation of compounds of this general type, but do not comment on yields or byproduct formation.
The patents cited give examples of the preparation of xanthogen formates according to the reaction: ##STR2## However, industrial processes have not been published or described.
The few articles on the reactions of alkali metal alkyl xanthates with alkyl chloroformates report that there are various competitive reactions which result in a reaction product which can contain impurities that are inert in the flotation process. These reactions consume a considerable proportion of the expensive starting materials, such as the alkyl chloroformates, increasing production costs.
The main articles published that describe these reactions are:
J. Welde, Journ.f.prakt. Chemie., (2) Band 15, p. 44 (1887) PA0 R. Holmberg, Journ.f.prakt. Chemie., (2) Band 71, p. 264 (1905) PA0 A. Cambron, Canadian Journal of Research, 2, p. 341 (1930) PA0 R. Sayre, J.Am. Chem. Soc., 74, p. 3647 (1952) PA0 R. Felumb, Bull. Soc. Chim. France, p. 890 (1957) PA0 S. M. Gurvich et al, Zh. Prikl. Khim, 39, (2) p. 531 (1966)
From these, different reaction conditions for reaction (1) result in a reaction product containing:
______________________________________ (I) Dialkyl xanthogen formate ##STR3## 0 to 80% (II) Dialkyl xanthic anhydride ##STR4## 10 to 100% (III) Dialkoxy carbonyl sulfide ##STR5## 0 to 40% (IV) Dialkyl carbonate ##STR6## 0 to 20% Unreacted raw materials and other impurities 0 to 10% ______________________________________
The reaction mechanisms that result in this product mix have been studied by Cambron, Op. cit. and S. M. Gurvich et al, Op. cit., and in work leading to the present invention.
Welde found that the uncontrolled reaction between potassium ethyl xanthate and ethyl chloroformate in water resulted in the production of only xanthic anhydride (II) and diethyl carbonate (IV). Holmberg used ice to cool the reaction and obtained a mixture of compounds (I) and (II), observing that a xanthate excess resulted in an increase in production of xanthic anhydride (II). Gurvich et al studied the reaction between potassium butyl xanthate and methyl chloroformate and found that COS was evolved and that the reaction product analysis varied as a function of the reaction conditions, yielding:
Compound (I): 60 to 100% PA1 Compound (II): 0 to 30% PA1 Compound (III): 0 to 8%
To explain this composition range they postulated the following reactions: ##STR7## They postulate that the intermediate reaction product of reaction (2) is unstable and loses COS to form the xanthic anhydride.
The mechanism proposed by Gurvich implies that the xanthate anion attacks the C.dbd.O group of the xanthogen-formate, but this group is less active than the C.dbd.S group. Further, reaction (4) is unlikely, as if it is done independently in the laboratory yields do not reach 30%. Based on this and the work reported by Cambron and current work, the more probable reaction mechanism is: ##STR8##
The production of compound (IV), COS and other impurities can be accounted for, in the presence of water, which is normally added to the reaction or produced in the systhesis of the xanthate, by the following reactions: ##STR9##
Several workers have found that high reaction temperatures favoured the production of compound (IV), which is not surprising as reaction (7) is the recommended synthesis for diethyl carbonate when carried out at 80.degree. C. Holmberg's observation that adding the chloroformate to the xanthate favoured the production of xanthic anhydride can be deduced from reactions (2) and (5).
Cambron (p. 346) describes an experiment which supports reaction (2) when xanthate is added to a xanthogen formate.